The solar energy industry is expanding at a rapid pace. Much of that expansion is due to increases in residential and small commercial photovoltaic (PV) installations. Increasingly these installations are directly connected to the utility grid without the use of batteries. Inverters are the power electronics equipment that converts DC electricity produced by PV panels (collectively a PV array) into AC required by the grid.
For a three-phase inverter to start producing power several variables need to be within programmed tolerances. Some of these tolerances are stipulated by Standard for Inverters, Converters, Controllers, and Interconnection System Equipment for Use with Distributed Energy Resources, UL 1741, Edition 1, May 7, 1999, which is incorporated herein by reference. One important start up variable is the input DC voltage. However, this voltage by itself can not tell you when a solar array is experiencing enough solar irradiance to start producing meaningful amounts of power. A PV array in low light conditions can produce a full voltage but still has very little power generating ability. The standard UL 1741 stipulates that an inverter must wait 5 minutes after sufficient DC voltage is detected to begin exporting power to the grid. If after an inverter connects to the grid and it is evident that the power generation level is not significant enough to justify continued operation, the inverter may choose to or be caused to shut down. Another 5 minute waiting period is required before the inverter can re-connect to the grid. This process of start up and shut down can continue for some time in certain situations, such as a cloudy morning. Repeated start up and shut down cycles can cause significant wear and tear on an inverter, especially since often the only moving part in an inverter may be the electrical contacts that connect and disconnect electrically from the grid. It would be advantageous to provide a start up algorithm which limited start up/shut down cycles while still allowing the inverter to connect to the grid in a timely manner when sufficient solar irradiance is present.